I don't really know about the "voltage proportional to speed" type of systems, so am unable to comment.
Regarding the vehicles I come into contact with usually, a pulse type system is normally employed and utilises the ABS wheel speed sensors, along with the steering angle sensor, to calculate road speed. Since the outer wheels rotate faster than the inner wheels during a turn, steering angle has to be taken into account and some form of averaging takes place. No single wheel speed sensor can be relied upon as a true indicator of speed, unless the vehicle is travelling in a straight line, with no slippage (Read: Wheel-spin or brake lock-up) The system uses a fixed number of pulses from the ABS wheel speed sensors and a programming option, to cater for differing tyre circumference, to allow for the numerous tyre sizes offered throughout the vehicle range.
Normally, there is an output from the ABS control module, either via a dedicated VSS wire or over the CAN-Bus. Usual places that the vehicle speed signal is routed to are:
ECM - Engine Control Module (Also known as PCM - Powertrain Control Module)
Instrument cluster - Quite obviously..
Radio/Infotainment module - Speed dependant volume control is common-place these days.
And lastly, but something that should not really be tapped into...The SRS (Safety Restraint System)
I'm afraid that the above info could be too manufacturer-specific to be applied as a steadfast rule to most makes, so the best option I can suggest, is to maybe subscribe to Autodata, where you would have access to info for a large number of makes, along with telephone support for those that haven't made the latest release.
HTH.
Do you need to use the Vss at all?, why not just calculate the distance travelled from the GPS data?.
we cannot afford the errors due to signal losses that may happen under bridges or in tunnels... so we have to compensate with another source of information
True. But for example, in Chicago here where i live, theres a LOT of underground tunnels and streets that don't see the surface for a few miles. Theres actually a few streets you can turn to while still underground.
How can you know where the person is going if he turns underground just by reading the vehile speed? Or are you planning on implementing an actual sensor to also know when they are turning and by how many degrees?
On OLD cars, it is rare to find a VSS signal, although my 20 year old bmw has one, on modern cars it can be found either on pin 1 of the ISO connector behind the radio or behind the speedo head. If your getting a voltage proportional to speed, it means that you tapped the wire before processing by the ECU, one of the other wires coming from the ECU will have a nice square wave proportional to speed. (could be 5v, could be 12v - depends on the vehicle)
NEW cars that have CAN enabled systems generally do not have a vss wire, but some do (toyota avensis, ford mondeo (MK3), etc)
It would appear though that you are trying to re-invent the wheel.
The type of people who would fit this gizmo for you will already have a source of convertors to use with your system. I just flicked though my files and found 12 different suppliers who I deal with regularly who can supply adaptors for ANY car on the market to a nice square wave pulse. They have adapters that clip on the mechanical speedo cables, they have CAN adapters, they have sensors with magnets for when nothing else works, that you attach to a driveshaft/propshaft.
I personally think you should just concentrate on making your gizmo, as there are already enough places that can supply the interface from a car to it. Plus you will be spending a load of cash making adapters that could be spend better else where - as some of your customers cars will need one, some wont, some will be can, others wont.
(Im a taximeter installer btw)
All taximeters (that I have ever dealt with) expect to recieve a square wave pulse input, the meters are tolerant of both 5v and 12v signals.
The meters will accept either a 5v/gnd, 12v/gnd or float/gnd input (i believe the last case is handled with a diode to prevent backfeed and a pullup resistor inside the meter. (when looking at the circuit with a testlamp in the last case you get GND_No Connection_GND_No Connection etc on the pulse wire.
No meter I have ever dealt with works directly with the analogue varying voltage from the speed sensor.
when we get cars in, we buy a meter from one of our suppliers, and it is our job to put in whatever adapters as neccessary to generate a nice plulse for the meter. In some cases this would include adding new sensors, pull-ups - neccessary for cars using abs sensors for speed, as the floating signal doesnt get pulled high fast enough - on these vehicles things work fine till about 50mph, then stop working at all unless an external pull-up is used.
From your side I would think your gizmo needs to handle the three pulse types above. Then the car is driven over a measured mile, dont know what the situation around the world is, but in the UK - EVERY local county has at least one 'Official' measured mile - its what the police cars use to calibrate there systems. anyway you reset the count at the beginning, drive the mile and record the count at the end. It can be anywhere from approx 1000 pulses to over 100,000 pulses. Store this in your system and use it for calculating distance, speed, etc.
hope this helps
yes/no answers
does your system need to handle analogue variable voltage NO
does you system need to handle the 3 pulsed arrangements above YES
will a professional installer be able to fit your product to ANY car, YES if the condition above is satisfied.
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